On asphalt plants it is desirable to have a variety of air pollution control measures. The asphalt making process, by its very nature of heating and processing the bituminous asphalt components, produces a considerable quantity of undesirable hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), particulate matter and other emissions which constitute the unfortunate signature plume of an asphalt plant, commonly referred to as "blue smoke." In addition to being a source of air pollution, asphalt plants are noisy and visually unappealing, owing to their network of open conveyors, hoppers, bins, blowers and other heating and material handling equipment. Accordingly, asphalt plants in general are regarded as quite a nuisance, especially in and around residential areas.
The typical asphalt plant has high energy requirements. The drum dryer/mixer typically includes a gas burner to dry the aggregate material and to heat the mixing zone to foster adequate mixing of the aggregate with the liquid asphalt. The asphalt material contained in the asphalt storage tanks must be constantly heated to maintain the asphalt cement in its liquid state, and thus another gas burner or similar heating system is required in order to constantly heat the storage tanks. Thus, burner emission are created at both the asphalt storage tanks and at the drum dryer/mixer.
Moreover, the volatile components of the heated asphalt cement as well as the finished asphalt create a certain amount of fugitive emissions as the asphalt components and the finished asphalt are stored, mixed, and transported through the plant. Furthermore, the asphalt cement storage tanks and the asphalt storage silos are usually vented in order to prevent undue pressure build up, especially on hot days, which further complicates the fugitive emission problem. Additional fugitive emissions are created when the finished asphalt material is loaded onto trucks for transport to a job site.
One approach to alleviating the fugitive emission problem has been to enclose portions or all of the plant in order to minimize the amount of leakage from the ductwork and conveyors in the plant. Such an approach, an example of which is described more fully in U.S. Pat. No. 5,620,249, does not provide an improved mitigation system and is typically best suited for applications in which the plant can be made very compact, which is not always feasible.
Attempts have also been made to apply flameless media burner technology to asphalt plants. Media burner technology uses a bed or matrix of ceramic materials which act as a flame arrestor, thereby controlling the rate and temperature of the combustion process. Externally mixed fuel is added to the media burner, which is preheated until a self-sustaining combustion is initiated. Ideally, a very efficient centralized media burner should be able to supply heat to the various process components, so that the maximum amount of energy is extracted from the consumed fuel. Unfortunately, existing media burner technology has proven unsatisfactory for asphalt processing plants. The externally mixed fuel components have proven to be too explosive for safe, everyday applications.
Accordingly, there exists a need for an improved asphalt plant that will produce significantly less process and fugitive emissions. There also exists a need for an improved asphalt plant having a centralized heat source incorporating safer and more reliable media burner technology. Such an improved media burner should function to supply all plant process heat requirements and should function to mitigate captured fugitive emissions.